I get the error message on the youtube video screen: "Your browser does not currently recognize any of the video formats available. Click here to view our FAQ about HTML5 video.

The FAQ page says what I need to supportit and then has green or red marks indicating what they detect I have. It says I need "Video tag" and one of h.264 or WebM. There is a green check next to "Video tag" and WebM" - so it should work. There is a red exclamation point by h.264.

I also tried installing the Adobe Flash player 11.6.602.171. It says it installed okay, and it's for my browser. The video still doesn't play.

Windows XP SP3. I tried disabling my firewall.

This is a two month old version of Firefox, but HTML5 is MUCH older. Firefox 10 supported it. I can't use a newer version of Firefox because my plugins aren't updated for it yet. But I doubt that's the problem.